The goal is to understand how normal mammary epithelial cells become preneoplastic, and what causes the preneoplastic cells to grow into tumors when exposed to the hormonal signals which should promote differentiation instead. We are designing our work to test how these abnormal developmental events take place. Becuase the development of the mammary gland and growth of mammary tumors are so strongly modulated by extracellular variables, methods have been devised for primary cell culture of the 3 cell types (normal, preneoplastic, tumor), so that hormone levels, nutritional status, cell-cell interactions, etc., can be controlled as stringently as possible. Our most interesting observations have been on tumor cells, which behave quite differently in primary culture than in typical cell lines. The dividing cells are unadhesive and become detached from the monolayer, a process reminiscent of metastasis. Only a small proportion (10-20%) of the cells in the monolayer are engaged in the cell cycle. We are now analyzing the growth properties of this released cell population, both in culture and in vivo. The next phase of the work, now beginning, is to study the modulation of differentiated mammary function (milk, virus synthesis) in culture. We are now able to promote lactose synthesis in cultures of normal mammary epithelium. Lactose is produced only in cultures exposed to the expected hormones (insulin, hydrocortisone, prolactin). We have detected newly-made lactose only in cultures "contaminated" with mammary fibroblasts. High doses of fetal calf serum (e.g., 15%) inhibit lactose synthesis. Once the control of lactose production in normal-cell cultures has been more carefully defined, we propose to study cultures of cells from hyperplastic alveolar nodules, to see if there are differences detectable in the control circuits of this preneoplastic cell population.